Method of growing a crystal of titanium dioxide



Oct. 20, 1970 N. E. FARB 3,535,077

METHOD OF GROWING A CRYSTAL OF TITANIUM DIOXIDE Filed March 28. 1966 REGION C REGION A REGION B INVENTOR. NORMAN E FARB BY j ATTORNEY United States Patent U.S. Cl. 23-202 4 Claims ABSTRACT OF THE DISCLQSURE A method of growing crystals of inorganic binary compounds in which a stoichiometric gradient is established and maintained across the crystal by providing a reactive atmosphere in contact with the crystal. Under the infiuence of this gradient, there occurs a steady state mass diffusion of formed crystal lattice defects to the surface of the crystal resulting in crystal growth.

The present invention is directed to methods of growing crystals of inorganic binary compounds and more particularly to methods of crystal growth and purification utilizing steady state mass diffusion of crystal lattice defects and impurities.

The growth of reasonable size crystals of ultra-high purity with low dislocation count and impurity inclusion is of vital importance to research into the intrinsic characteristics of crystals.

The present invention is related to the solid phase crystal growing methods of the prior art and utilizes steady state mass diffusion in inorganic binary compounds to obtain crystal growth or regrowth. In the present invention the process by which the atomic species involved arrive at the growing surface is significantly different than the prior art crystal growth techniques. Thus, one of the atomic species involved in the growth is initially chemically combined with the crystal. Further, the method of the present invention utilizes diffusion through the crystal specimen. Further, the method of the present invention utilizes diffusion by either vacancies or interstitials or both depending upon the nature of the crystal species involved.

Therefore, it is the principal object of the present invention to provide a method for growing crystals by controlled steady state mass diffusion.

It is another object of the present invention to provide a method of growing crystals of relatively higher purity and perfection.

It is a further object of the present invention to provide a method of growing crystals utilizing controlled stoichiometric gradients in binary single crystals.

It is a still further object of the present invention to provide a method for increasing the density of polycrystalline material.

It is another object of the present invention to provide a method for reducing the impurity content in grown crystals.

These and other objects of the present invention will become more apparent from the following detailed description of various embodiments of the present invention taken together with the drawings, hereby made a part thereof, in which:

FIG. 1 is a sectional view of the apparatus for carrying out the method of the present invention; and

FIG. 2 is a side view of the sample depicting the various regions resulting from the steady state diffusion method of the present invention.

One feature of the present invention is to utilize a binary crystal of the form M X where M is a metal and X is a combining species, i.e., non-metal including metalloid, in

which crystal growth takes place at one or more surfaces under the influence of a stoichiometric gradient. A stoichiometric gradient, i.e., a composition or concentration gradient, is generated and the steady state mass diffusion of crystal lattice defects such as interstitials or vacancies is controlled so that the growth occurs when lattice defects associated with the stoichiometric gradient are present. The method of the present invention is applicable to binary crystal compounds in which the flux of one of its species in respense to a stoichiometric gradient is sufficiently rapid to permit growth on a practical time scale. Since the time required for growth is inversely proportional to the degree of non-stoichiometry of the compound, i.e., the composition or concentration gradient, and the diffusion coefficient of the defects that migrate in response to the stoichiometry gradient, compounds in which these quantities are relatively large offer the best areas for process use. More specifically, the method of the present invention is directed to the growth of inorganic binary crystals M X where M, the primary species, is a metal, e.g., Ti, Zn, Cu, Cd, U, Zr, Pb, Ba, etc., the secondary species, is a non-metal, including metalloids, e.g., O, S, Se, Te, halides, etc.

Another feature of the present invention is the regrowth of inorganic binary crystal compounds in which opposite surfaces of a single or polycrystalline membrane are subjected to oxidizing and reducing atmospheres, since .such a combination of atmospheres produces a stoichiometric gradient and the lattice defect gradient associated with it. The oxidizing atmosphere is produced by a relatively high partial pressure of gaseous non-metal, including metalloid atoms or molecules. A reducing atmosphere can be produced by two methods: (1) utilizing a very low partial pressure of the non-metal including metalloid, or (2) utilizing a high partial pressure of metal atoms. Thus, the reducing atmosphere produces interstitials on the reduction side if the crystal becomes nonstoichiometric by the production of cation interstitials, or the oxidizing atmosphere produces cation vacancies on the oxidizing surface if the crystal becomes non-stoichiometric by production of cation vacancies, or a combination of such effects occurs. In any event, the lattice defects involved diffuse through the crystal, resulting in a continued cation supply to the oxidizing surface. This cation supply then forms crystalline growth by oxidation with the oxidizing gas. This type of growth or regrowth is referred to hereinafter as endotaxial growth.

A further feature of the present invention is the enhancement of crystal purity by the preferential steady state diffusion of impurities. These impurities which have a diffusion coefficient less than the principal cations will not diffuse through the crystal as rapidly and higher purities will result in the new growth region. Also, the ones which have a greater diffusion coefficient will preferentially migrate to the front section of the crystal which may then be removed.

Referring now to the drawings in detail, the specimen 10 of FIG. 1 has a surface 1 exposed to a preselected atmosphere at pressure p and surface 2 exposed to a preselected atmosphere at pressure p The specimen is heated to a preselected temperature as described in detail hereinafter. Four main cases will be considered, that is, those binary systems which exhibit only one predominant type of non-stoichiometricatomic defect. The system considered is the metal non-metal (including metalloid) crystal system of the form M X Further, each of the four main cases have two different combinations of gaseous atmospheres that produce either crystal growth or crystal regrowth. Combination one has a high partial pressure of non-metal, including metalloid, gas atoms or molecules on one surface and a high partial pressure of metal gas atoms or molecules on the other surface. Combination two has a high partial pressure of non-metal or metalloid gas atoms or molecules on one surface and a low concentration of non-metal, including metalloid, gas atoms or molecules on the other surface. The system considered is the metal non-metal (including metalloid) crystal system of the form M X In the metal atom interstitial case involving gas, combination one, the metal atom imperfections are moving and result in a mass diffusion of metal atoms from the surface which is in equilibrium with the high partial pressure of gaseous metal atoms to the surface of high partial pressure of the non-metal or metalloid. The resulting concentration of the metal atoms due to diffusion is therefore higher than the equilibrium concentration at the non-metal, including metalloid, high partial pressure surface and the excess interstitials react with the non-metal gas atoms so that the crystal grows on the non-metal, including metalloid, high partial pressure surface. The high metal partial pressure surface continues to supply metal atom interstitials in an attempt to maintain equilibrium at this surface. Crystal growth occurs.

In the metal atom interstitial case involving gaseous combination two the metal atom imperfections are moving and result in a mass diffusion of metal atoms from the surface which is in equilibrium with a low partial pressure to the surface where the partial pressure is higher. The resulting concentration of the metal atoms due to diffusion is therefore higher than the equilibrium concentration at the high partial pressure surface and the excess interstitials react with the gas atoms so that the crystal grows on the high partial pressure surface at the expense of the low partial pressure surface. The low partial pressure surface continues to supply metal atom interstitials in an attempt to maintain equilibrium at this surface.

In the metal atom vacancy case involving gaseous combination one the metal atom near the oxidizing surface is promoted to the surface of the crystal by thermal stimu lation. The crystal in the presence of a selected gas has a certain fraction of its surface sites occupied by these gas atoms through chemisorption or adsorption and thus can react and form a unit cell of metal and gas atoms. The process can also occur by a collision of a gas molecule with the surface metal atom. The high concentration of metal atom vacancies on the surface adjacent the high non-metal, including metalloid, gas diffuses toward the opposite surface where the equilibrium concentration of vacancies on that surface is now imbalanced. This results in annihilation of the metal atom vacancies by the adsorped metal atoms. Thus, crystal growth results on the high non-metal, including metalloid, pressure side, there is mass diffusion of cations in the cation sublattice through the seed crystal, non-metal, including metalloid, gas molecules are chemically combined to unit cells on the high non-metal, including metalloid, surface, and gaseous metal atoms annihilate the vacancies on the high metal gas pressure surface.

In the metal atom vacancy case involving gaseous com bination two the metal atom near the surface is promoted to the surface by thermal stimulation. The crystal in the Atmosphere presence of a selected gas has a certain fraction of its surface sites occupied by these gas atoms through chemisorption or adsorption and thus can react and form a unit cell of metal and gas atoms. The process can also occur by a collision of a gas molecule with the surface metal atom. The high concentration of vacancies on the surface adjacent the gas diffuses toward the opposite surface and is now imbalanced. This results in annihilation of the metal atom vacancies and the evolution of gas molecules. Thus, crystal growth results on the high pressure side, there is mass diffusion of cations in the cation sublattice through the membrane, gas molecules are deposited on the high pressure side, and gas molecules are released on the low pressure side.

In the non-metal interstitial case involving combination one crystal growth does occur on the high metal gas pressure surface. The non-metal interstitials diffuse from the surface in contact with the high non-metal pressure surface to the surface in contact with the high metal pressure surface. The interstitial non-metal atoms react with chemi-adsorbed metal atoms forming unit cells on this surface and crystal growth.

In the non-metal interstitial case involving combination two the crystal does not grow but merely acts as a pervious membrane to the non-metal gas.

In the non-metal vacancy case involving combination one the crystal does grow on the high pressure metal gas surface. The non-metal atoms close the high metal pressure surface, are promoted to the surface by thermal stimulation, and combine to form unit cells on the surface. The resulting concentration gradient of non-metal atom vacancies causes mass diffusion of non-metal vacancies to the surface in contact with the high non-metal partial pressure where they are annihilated by adsorbed or ehemi-adsorbed non-metal gas atoms on this surface.

In the non-metal vacancy case involving combination two the crystal does not grow.

Four additional cases of less significant crystal growth can occur in those crystal systems where the stoichiometric partial pressure may be spanned by the two partial pressure atmospheres. Under these conditions two crystal lattice defects occur at the two surfaces with a resulting diffusion of each defect towards the other surface. The resulting possibilities are summarized in Table I and Vacancies Anion Case l2 Ca Iuterstitials. Y Anion Case 13. Case 15, Vacancies N N N=erystal growth unlikely, Y=crystal growth.

TABLE II (species and Growth on Diffusion species DiiIusion species See Table I surface) surface 1 Growth on surface 2 from surface 1 from surface 2 1 (Non-metal (2) Yes N0 fetches) Metal atom interstitial. Case "1M .al (2) Ycs No (uouetcl1ing).. a. Do. C 9 {Non-metal (2) Ycs N o (etches) Vacancy (metal) use "ln letal (2) Ycs No (non-ctcl1ing) do C 3 {Non-metal (2 No. N

Metal (2) 5 N0. ges. Non-metal (2 0 Case 4 "{Mctal (2) Ycs Case 9 $3; 0 C 10 [Non-metal N0 Non-metal interstitial D0. lMctal 2 Yes .d0 Do. 0 11 tNon-metal( N0 (etches). Vacancy (metal) Metal atom interstitial.

ase "lMetal (2) Y s. No (non-etching) do Do. 12 Non-metal (2). Ycs No. Non-metalinterstitial Do. Metal 2 Yes Yel Do.

In the practice of the method of the present invention either a polycrystalline or single crystal specimen may be utilized. The time required for crystal growth, however, may be different because of the generally faster diffusion along grain boundaries.

The temperature used is generally above one-half of the melting temperature of the particular crystal so that diffusion can proceed and surface reaction rates are sufficiently fast. Various partial pressures may be used on either side of the specimen. The partial pressure span may be large and include the stoichiometric partial pressure or be smaller and on either side of the stoichiometric pressure. Increasing temperature is equivalent to changing the pressure, therefore, a variation in results will occur in particular systems with a change in temperature.

The process may be utilized on all ordered crystals including intermetallics or ordered metal alloys. When conduction is by ionic diffusion the surfaces must be electrically connected so that electronic conduction occurs externally between the metal partial pressure surface and the non-metal, including metalloid, partial pressure surface.

Various non-metal species may be utilized including halides, sulfides, tellurides, selenides, etc. The ratio of the thickness to the diameter of the specimen should be sufficiently small so that one dimensional diffusion will occur. However, it is within the purview of the present invention to utilize a three-volume (pressure) gas system.

Referring now to FIG. 1 in detail, the diffusion apparatus holds a thin, approximately 1 mm., crystal disk 10 between two volumes 12 and 14 having different atmospheres at controlled temperatures. Thermocouples may be placed on either side of the sample and the atmospheres monitored by a precision volume-measuring U tube if desired.

The gases used in the following examples were high purity grade CO, H H O, argon and C0 The gas flow rates were controlled by calibrated flow meters. The specimens were oriented by X-ray back reflection techniques and sliced with a diamond saw precision wafering machine. The specimens were then .ground and polished by conventional optical lapping techniques to size (fiat and parallel to less than four fringes) and then ground to a circular shape in a special lathe chuck.

Platinum washers 16 were prepared from 9.9 mil sheet stock with a 0.750 in. OD and a 0.400 ID. The ceramic rings 18 were cut and ground from high purity, high density alumina plates to a thickness of 0.0 to 0.2 mil less than that of the specimen with a 0.750 OD and a 0625 ID. The specimens were then placed inside the ceramic ring 18 and two platinum washers 16 were placed on either side. This arrangement was then loaded between the two alumina atmosphere-containing tubes 20 and 22 and placed in a furnace.

The samples 10, in the following examples of the preferred embodiment, were prepared from 99.99 percent pure TiO powder. Polyvinyl alcohol was used as a binder. The mixture was then dried and formed into a boule and compressed inside a flexible container at 20,000 lbs./sq. in. in a pressure vessel. The formed boule was then heated at 300 C. for 48 hours and then slowly brought up to 1250 C. in a helium atmosphere and allowed to sinter to 94.5 percent theoretical density for 72 hours. The sample disks 10 were then prepared in the manner described above.

EXAMPLE I Polycrystalline samples of 94.2 percent theoretical density were prepared and polished flat to within 2 to 4 fringes. The sample thickness was 0.76 mm. The alumina ceramic ring was prepared to a thickness of 0.000 to 0.005 mm. les than the thickness of the sample. A (TO/CO gas ratio equal to one and pure 0 were used in volumes 12 and 14 respectively. Both gases were at one atmospheric pressure.

An initial run at 1000 C. for four hours led to nonstoichiometric diffusion etching on the CO/CO surface. No growth was visible on the 0 surface. However, a cross-section revealed that densification was occurring in the center of the membrane. A second sample was then run for 32 hours at 1100 C. Densification and growth to and beyond the original 0 surface occurred.

Five distinct regions exist in the membrane as a result of the stoichiometry diffusion gradients. These are depicted in the sketch of FIG. 2. Region A is an area of regrowth beyond the original surface (shown as a dotted line). Region B is an area of densification of the polycrystalline material. The difference in thickness of this region probably reflects local variations in original density. Region C is an area of densification under the platinum seal 16 extending up the inside surface of the ceramic seal 18. Region D is an area essentially unscathed. Region B is a scale area that separated from Region B due to the dynamics of the diffusion.

EXAMPLE II Single crystal TiO was machined and optically polished (to within 2-4 fringes) to a thickness of 0.61 mm. Ceramic rings were prepared with a thickness 0.000 to 0.005 mm. less than that of the TiO disks. The samples were chemically polished in fuming H 50 for a minimum of three hours to remove the work hardened surface. CO/CO and O gases were used. The samples were run at temperatures of 900 C., 1000 C., 1000 C., and 1100 C., for 8, 6, 8, and 6 hours, respectively. All samples showed single crystal regrowth on the 0 surface. All samples showed non-stoichiometric diffusion etching in channels along the c-axis direction on the CO/CO side. Growth layers on this surface are nucleated in many positions and extend in the c-axis direction. The 1000 C. samples show a slightly changed surface growth condition resulting in fewer nucleation steps and a smoother surface. This could be due to higher surface mobility and/ or more uniform diffusion of titanium interstitials to the surface.

Crystal growth occurred between the A1 0 ceramic ring and the TiO membrane disk. The original CO/CO exposed surface showed more non-stoichiometric diffusion etching than those areas originally covered by the platinum seal. Typical samples showed that the regrowth region had grown above the 0.248 mm. thick platinum marker.

EXAMPLE III The crystals were oriented by X-ray techniques, machined and polished flat to within 2 to 4 fringes. The crystal membranes were oriented with the (011) crystallographic direction parallel to the diffusion direction. The initial run was performed in one atmosphere of CO and one atmosphere of 0 at 1150 C. for 96 hours on a 1 mm. thick sample. The exposed region of the crystal showed regrowth to a distance 1.5 mm. beyond the original 0 surface. Portions of the original CO exposed surface remained at the original level. Non-stoichiometric diffusion etching permeated the entire exposed region so that the sample was pervious to gas flow. X-ray analysis showed the crystallographic orientation to be predominantly c-axis. The sample showed a profusion of flat ribbon whiskers and circular whiskers. The time and tem perature was then reduced, and a 1 mm. sample was exposed to CO and O atmospheres for 23.5 hours at 962 C. As in all cases, the O -platinum seal is bonded to the sample and fracture of the original surface occurs when removal is attempted. One sample was not chemically etched in fuming H 50 and large areas of the exposed surface remained intact. The area under the platinum seal was attacked, however, and the greater portion of the titanium interstitial diffused from this region.

An outer ceramic ring was not used in this example so that the crystal grew beyond the original diameter of the circular membrane. Thus, there was no possibility that the crystalline regrowth occurred by surface diffusion from one surface to the other. The initial diffusion to the O platinum seal results in some diffusion to the unexposed surface. Thus, the region around the inner edge of the platinum seal on the 0 side always shows greater regrowth. However, as soon as the platinum seals to the TiO disk, the majority of the diffusion would be volume diffusion.

EXAMPLE IV One c-cut sample was diffused by using a CO atmosphere on one side and a partial pressure of 20 microns 0 for 24 hours at 965 C. The growth on the 0 surface was predominantly single crystal with some polycrystalline formation on the outer layer of the regrowth surface which extended 0.11 mm. below the original 0 surface. The non-stoichiometric diffusion etching was quite uniform to a depth of 0.4 mm. to 0.5 mm. A comparison emission spectrographic analysis of material removed from the two surfaces by a diamond tipped tool was performed. The results are shown in Table III.

TABLE III CO/COQ surface 02 surface titanium major Percent titaniumma ior Percent;

Aluminum 0. 00l 0. 01 Aluminum 0. 001 0. 01 Copper 0. O00l 0. 001 Copper 0. 0001 0. 001 Calcium 0. 0001 0. 001 Calcium 0. 0O01- 0. 001 Iron 0. 00O5 0. 005 Not detccted Silvcr 0. 000l- 0. 001 Not detected.

Magnesium 0. 0002 0. 002 Not detected No other metals detected Iron, silver and magnesium are thus removed from the grown crystal due to the titanium interstitial gradients. Cu, Ca, and Al, however, did not diffuse to the O surface. This strongly suggests that Fe, Ag, and Mg are interstitial foreign impurities, whereas, Cu, Ca, and Al reside in substitutional positions.

EXAMPLE V The non-stoichiometry produced by hydrogen reducing gases occurs at lower temperatures and is believed to form an OH bond with an O. This could have an effect upon the diffusion of Ti interstitials by lowering the concentration gradient because of a large number of H interstitials. Two experiments were performed using I claim:

1. A method of growing a crystal of titanium dioxide comprising the steps of selecting a seed crystal of titanium dioxide having at least a pair of substantially parallel opposite sides, establishing a stoichiometric gradient across said opposite sides of said seed crystal by heating said crystal to an elevated temperature between above onehalf of the melting temperature of the crystal and below its melting temperature while providing a reactive reducing or oxidizing atmosphere in contact with at least one of said sides so as to generate crystal lattice defects in said crystal by chemical reaction therewith, and maintaining said conditions of temperature and reactive atmosphere for a sufiicient period of time so as to maintain said gradient so that there is a steady state mass diffusion of crystal lattice defects of at least one defect species of titanium or oxygen to at least one side of said seed crystal whereby crystal growth occurs on at least said one side of the crystal by chemical reaction of the defect species of at least titanium or oxygen with the opposite one of said species to form titanium dioxide regrowth crystal.

3. The method of claim 1 wherein said stoichiometric gradient is established and maintained by subjecting said opposite sides of said seed crystal of titanium dioxide to a combination of preselected atmospheres selected from gaseous 0 CO, H H O, argon and CO at a temperature between 885 and 1100 C. for a period of time between 4 and 96 hours.

3. The method of claim 1 wherein said stoichiometric gradient is established and maintained by subjecting said one side of said TiO seed crystal to a gaseous oxiding atmosphere and the other side to a gaseous reducing atmosphere.

4. The method of claim 3 wherein said oxidizing and reducing atmospheres are at different pressures.

References Cited UNITED STATES PATENTS 3/1965 Allegretti et al. 23-223.5 XR 8/1967 Levine et al. 23-305 OTHER REFERENCES EDWARD STERN, Primary Examiner US. Cl. X.R. 

